This invention relates to colored magnetic particles for magnetophoretic display and a method for manufacturing the same, and more particularly to colored magnetic particles used for a magnetophoretic display panel which is adapted to carry out display using magnetophoresis or magnetic migration and a method for manufacturing such colored magnetic particles.
A magnetic drawing panel which has been conventionally known as a picture drawing board for infants in the art is generally constructed in such a manner that a honeycomb core formed with a number of hexagonal cells is sealedly arranged between a transparent or semitransparent drawing plate or base plate and a face member arranged opposite to the drawing plate and the cells each have a plastic dispersion liquid which contains magnetic particles sealedly received therein.
In the conventional magnetic drawing panel thus constructed, a magnet pen is brought into contact at a tip end thereof with a drawing surface of the panel, to thereby permit a magnetic field to act on the plastic dispersion liquid, resulting in the magnetic particles rising to the drawing surface, so that a desired picture or character may be displayed on the drawing surface due to contrast between a dispersion medium for the dispersion liquid and the magnetic particles. Also, movement of an erasing bar sildably arranged under the face member permits the magnetic particles which have risen to sink in the dispersion liquid, to thereby erase the picture or character displayed.
The plastic dispersion liquid conventionally used for this purpose is generally obtained by compounding magnetic particles, a thickener containing fine particles and a coloring agent in a dispersion medium. The dispersion medium may be selected from the group consisting of, for example, polar dispersion mediums such as water, glycols and the like and non-polar dispersion mediums such as organic solvents, oils and the like. The magnetic particles may be selected from the group consisting of particles of magnetic oxide materials such as black magnetite, xcex3-hematite, chromium dioxide, ferrite and the like and particles of magnetic metal materials such as alloys of iron, cobalt, nickel and the like. The fine particle thickener may be selected from the group consisting of finely divided silicate such as silicic acid anhydride, hydrous calcium silicate, hydrous aluminum silicate, silica powder, diatomaceous earth, kaolin, clay, bentonite or a mixture thereof; alumina; ultra fine calcium carbonate; ultra fine activated calcium carbonate; ground calcium carbonate; hydrous basic magnesium carbonate; barium sulfate; and the like. The coloring agent may be selected from the group consisting of a white pigment, a colored pigment, a dye and the like. Use of the plastic dispersion liquid permits a figure, a character or the like to be displayed on the drawing surface due to contrast between a ground color of the white or opaque white dispersion medium and a line drawn by the black magnetic particles.
As a new display panel following the above-described magnetophoretic display device wherein a black line is drawn on a white ground, it is desired to develop a magnetophoretic display device which is constructed so as to exhibit color display. The inventor as well made a study of realizing such a color magnetophoretic display panel. Techniques which have been conventionally proposed for this purpose include techniques of coloring a metal powder, grinding techniques and spray drying techniques.
The grinding techniques are constructed so as to mix iron powder as magnetic particles, a pigment and synthetic resin with each other to prepare a mixed material, dry the material and grinding it to form it into particles having a uniform particle diameter. The grinding techniques permit the material to have a bright color. However, the techniques cause the pigment to be peeled from the material during the grinding step, to thereby produce fine powder, leading to contamination of a dispersion liquid. The spray drying techniques include the step of mixing iron powder as magnetic particles a pigment and synthetic resin together to prepare a liquid mixture and the step of spraying the mixture in a hot air atmosphere to form particles of a required particle diameter. Thus, the spraying techniques are free from any grinding step, to thereby eliminate peeling of the pigment from each of the particles due to the grinding. However, the spraying techniques cause peeling of the pigment from the particles due to striking of the particles against air during the spraying. Further, they cause the pigment peeled from the particles to form undesired particles.
Also, even when the prior art permits colored magnetic particles which prevents peeling of the pigment from the particles during manufacturing thereof to be provided, the colored magnetic particles thus provided cause a coloring layer to be gradually peeled from each of the particles when they are sealedly received together with a plastic dispersion liquid in a magnetophoretic display panel, because the magnetic particles collide with each other during repeating of rising and sinking thereof in the dispersion liquid during use of the magnetophoretic display panel. This results in the coloring pigment peeled from the coloring layer dissolving in the dispersion medium over a long period of time, leading to contamination of the dispersion medium.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide colored magnetic particles for magnetophoretic display which are capable of substantially preventing peeling of a coloring pigment from magnetic particle elements.
It is another object of the present invention to provide colored magnetic particles for magnetophoretic display which are capable of preventing contamination of a dispersion medium.
It is a further object of the present invention to provide an improved method for manufacturing said colored magnetic particles for magnetophoretic display.
In accordance with one aspect of the present invention, colored magnetic particles for magnetophoretic display which are contained in a plastic dispersion liquid for a magnetophoretic display panel are provided. The colored magnetic particles include: magnetic particle elements; a coloring layer formed on each of the magnetic particle elements from a first dispersion liquid using a coating apparatus of a tumbling, fluidizing and granulating type, to thereby color each of the magnetic particles, the first dispersion liquid being constituted of a coloring pigment, synthetic resin and water, the synthetic resin being in the form of an aqueous emulsion; and a protective layer formed on the coloring layer, the protective layer being formed from a second dispersion liquid constituted of synthetic resin dispersed in water.
In a preferred embodiment of the present invention, each of the colored magnetic particles further includes a white primary coat formed under the coloring layer, wherein the white primary coat is formed from a third dispersion liquid constituted of titanium white, synthetic resin and water on each of the magnetic particle elements by spraying the third dispersion liquid in the form of mist-like droplets in the coating apparatus.
In a preferred embodiment of the present invention, the colored magnetic particles contain the magnetic particle elements of 75 to 77 wt %, the pigment of 13 to 14 wt % and the synthetic resin of 9 to 10 wt %.
In a preferred embodiment of the present invention, the colored magnetic particles have a saturation magnetization of 80 to 150 emu/g and a magnetization, when a magnetic field of 1 kOe is applied thereto, of 35 to 65 emu/g.
In a preferred embodiment of the present invention, 90 wt % or more of the colored magnetic particles have a particle diameter of 50 xcexcm to 130 xcexcm.
In a preferred embodiment of the present invention, the magnetic particle elements have an apparent density of 2.3 to 3.0 g/cm3 and a saturation magnetization of 130 to 200 emu/g.
In accordance with another aspect of the present invention, a method for manufacturing colored magnetic particles for magnetophoretic display which are contained in a plastic dispersion liquid for a magnetophoretic display panel is provided. The method includes the steps of: forming a coloring layer on each of magnetic particle elements from a first dispersion liquid using a coating apparatus of a tumbling, fluidizing and granulating type, to thereby color each of the magnetic particles, the first dispersion liquid being constituted of a coloring pigment, synthetic resin and water, the synthetic resin being in the form of an aqueous emulsion; and forming a protective layer on the coloring layer, the protective layer being formed from a second dispersion liquid constituted of synthetic resin dispersed in water.
In a preferred embodiment of the present invention, the method further includes the step of forming, prior to the forming step of the coloring layer, a white primary coat from a third dispersion liquid constituted of titanium white, synthetic resin and water on each of the magnetic particle elements.
In a preferred embodiment of the present invention, the forming of the coloring layer is carried out by spraying the first dispersion liquid in the form of mist-like droplets in the coating apparatus.
In a preferred embodiment of the present invention, the forming of the protective layer is carried out by spraying the second dispersion liquid in the form of mist-like droplets in the coating apparatus.
In a preferred embodiment of the present invention, the forming of the white primary coat is carried out by spraying the third dispersion liquid in the form of mist-like droplets in the coating apparatus.
In a preferred embodiment of the present invention, the first dispersion liquid is coated on the magnetic particle elements at a product temperature lower than a glass-transition temperature of the synthetic resin.
In a preferred embodiment of the present invention, the magnetic particle elements have an apparent density of 2.3 to 3.0 g/cm3 and a saturation magnetization of 130 to 200 emu/g.
In a preferred embodiment of the present invention, the pigment has a particle diameter of between 0.01 xcexcm and 6 xcexcm.